Experimental Characterization and Modeling of the Performance of a

Jan 5, 2007 - using our own facilities, to evaluate the hydraulics and mass- ..... Note that the irrigation density used in this study is well below...
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Ind. Eng. Chem. Res. 2007, 46, 883-893

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SEPARATIONS Experimental Characterization and Modeling of the Performance of a Large-Specific-Area High-Capacity Structured Packing Z ˇ . Olujic´ * and M. Behrens Laboratory for Process Equipment, Delft UniVersity of Technology, 2628 CA Delft, The Netherlands

L. Spiegel Sulzer Chemtech, P. O. Box 65, 8404 Winterthur, Switzerland

This paper presents the results of a comprehensive experimental study performed with a Sulzer high-capacity structured packing of a larger specific area, which provided a basis for validation of the Delft model. Purely geometry-based adaptations were made to the Delft model, to account properly for the effect of short smooth bends on both ends of each corrugated sheet, which, in turn, seemed to be beneficial for capacity without adversely affecting the efficiency. Comparisons indicate fairly good agreement with experiments as well as with the predictions of an in-house empirical model contained in Sulzer’s software package, Sulpak. Introduction Corrugated-sheet structured packing is a well-established vapor/liquid contacting device used predominantly in distillations below and around atmospheric pressure. Pressure drop and/ or mass-transfer efficiency of these packings can be predicted with some degree of confidence, using some of the generalized predictive models from the literature.1-5 All these methods rely on two or more empirically determined packing-type and sizespecific constants. This limits the usefulness of these methods with mainly an academic background; moreover, to extend their applicability, at least one hydraulic experiment and/or one total reflux experiment for each type and size of packing is required. This currently is not very easy, with only a few of the suitable test facilities being available for these purposes (these are used mainly for proprietary tests). A more practical approach, at least for a manufacturer of packings generating relevant data using own test facilities, as is the case with Sulzer Chemtech, was to rely on a proven empirical model.6 Sulzer’s empirical knowledge that has been gathered and improved over the decades is summarized in a proprietary software package known as Sulpak,7 which has been made available to customers for their column (re)design purposes and to universities to be used in column design courses. Newer versions of this package include also a new generation of Sulzer high-capacity packings, belonging to the so-called MellapakPlus family. The overall performance characteristics of common-sized MellapakPlus packings are reported in some conference papers,8-10 as well as in a journal paper that addressed the hydrodynamics of MellapakPlus 252.Y.11 A more detailed discussion of the approaches to increasing the capacity of structured packings can be found elsewhere.12-14 According to the Sulzer tests that were conducted in a 1-mdiameter column with a chlorobenzene-ethylbenzene (CB/EB) system,8,9 a capacity increase of 40% was observed under * To whom correspondence should be addressed. Tel.: +31 15 278 6674. Fax: +31 15 278 6975. E-mail: [email protected].

vacuum conditions (100 mbar), which decreased to ∼25% at almost atmospheric pressure. Generally, the hydraulic performance of high-capacity packings cannot be predicted using any of aforementioned methods developed for conventional structured packings. However, for MellapakPlus packings, predictions can be made using new versions (Version 3 and higher) of the software package Sulpak. On the other hand, these total reflux experiments indicated that the mass-transfer efficiency of new packing is practically equal to that of the conventional counterpart, and even more stable in loading region, persisting up to the point of onset of flooding. A generalized approach to the modeling of the performance of corrugated-sheet structured packings is embodied in the socalled Delft model,15-17 which correlates, in a fundamentally sound way, the relationship between the fluid dynamics imposed by a highly ordered geometry and the pressure drop and masstransfer performance. The model was validated using experimental data obtained with several conventional Montz packings, including those with a specific area of 400 m2/m3 under total reflux conditions.18 These tests, as well as the most recent one,19 including both conventional and high-capacity structured packings with a specific area of 350 m2/m3, indicated that packings with a large specific area make less-efficient use of the installed area than the common size packings. This was taken into consideration accordingly by an empirical correction term, and, as demonstrated in a recent paper,20 this intervention increased the overall predictive accuracy of the Delft model appreciably. It should be noted here that the Sulzer data for the CB/EB system indicate no loss of efficiency with increasing specific area.8 This means that, for Mellapak packings in conjunction with the CB/EB system, there is no need to apply the effective specific area correction expression that is available in the Delft model.20 Being independent of the packing type and size, the Delft model seems to be flexible enough to account appropriately for the effects of changes in geometry of the flow channels, such as those encountered in high-capacity structured packings.13

10.1021/ie051146f CCC: $37.00 © 2007 American Chemical Society Published on Web 01/05/2007

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Table 1. Delft Model Equations parameter

expression

(A) Pressure Drop overall pressure drop

∆p ∆p ) ∆z ∆z

( )

preload

1

Fload

preloading region pressure drop

∆p ∆z

( )

preload

) (σGL + σGG + σDC)

FGuGe2 2hpb

2

pressure drop enhancement factor for the loading region

Fload ) 3.8

( ) ( ) FG FG,lp

2/(sin R)

uLs

0.13

3

2

 gdhG

(B) Liquid Holdup liquid holdup in the loading region

hload ) hpreload +

[

0.2 1 + 150

) ](

(

∆p/∆z FLg

2

)

uLs2ap g sin RL

0.25

4

liquid holdup in the preloading region

x 3

hL,preload ) apδL ) ap

3uLsµL apFLg sin RL

5

(C) Mass-Transfer Efficiency height of packed bed equivalent to a theoretical plate (equilibrium stage)

(λln-λ1)

6

HETP ) HoG

height of gas side overall mass transfer unit

HoG ) HG + λHL )

( )

uLs uGs +λ kGae kLae

7

stripping factor for total reflux (L/G ) 1) situation

λ)m)

R dy ) dx [1 + x(R - 1)]2

8

The purpose of this study is twofold. The first objective is, using our own facilities, to evaluate the hydraulics and masstransfer performance of the Sulzer high-capacity packing with a larger specific surface area, which is frequently used in fine chemicals and cryogenic air distillations. The second objective is to use the experimental data to evaluate the Delft model, with regard to its predictive accuracy, and compare it to the Sulzer predictive model that is contained in the software package Sulpak.7 Predictive Models Delft Model. A summary of the main equations of the Delft model is given in Table 1. Characteristic correlations for the individual components of these equations, developed and presented in more-or-less complete and correct form in earlier papers,15-17 including the correction for the increased loss of interfacial area that is experienced with large specific areas, can be found elsewhere.20 The present paper considers only the adaptations made in the model to describe the effect of the specific geometry features of MellapakPlus 752.Y properly. Figure 1 shows a side view of a sheet of MellapakPlus 752.Y (proprietary name) packing, which is characterized by smooth bends on both ends of each sheet and a moderately embossed

surface with a regular pattern of 4-mm-diameter holes. The main corrugation-related dimensions, based on the nominal surface area, are summarized in Table 2. It should be noted that the corrugation angle of this packing is ∼41°, which is well below the commonly observed value of 45°. Regarding the fact that the bends on both ends of MellapakPlus 752.Y are rather short, the main body of the corrugated sheet is practically unaffected by this modification. With respect to the nature of the Delft model, this means that only the hydraulic portion of the model that describes the pressure drop, in conjunction with the onset of loading at the transition between packing elements, must be considered. More precisely, the main concern here is the pressure drop caused by the change in direction at the transitions between packing elements. In the overall pressure drop expression of the Delft model (eq 1 in Table 1), a distinction is made between the preloading and loading regions. As indicated by the corresponding subscripts in eq 2 (Table 1), the pressure drop in the preloading region consists of three main contributions, i.e., the losses due to the interaction of vapor and liquid (GL), the interaction of crossing gas streams at the intersections of oppositely oriented vapor flow channels (GG), and the losses associated with frequent changes in the direction of vapor flow (DC). The latter is actually a zigzag flow, with sharp bends at each transition between packing elements, which is, in the present case, effectively avoided by introducing short bends at the ends of corrugated sheets. It is important to note here the fact that the corresponding “friction factors” for two main sources of the pressure drops the gas-gas interaction and the flow direction changesare both a strong function of the corrugation (the vapor flow channel) inclination angle. For gas-gas interaction, this becomes the corrugation angle of MellapakPlus 752.Y, i.e., 41°. The same angle is used in eq 3 (see Table 1) to describe the extent of deviation of the pressure drop in the loading region from that of the preloading region.21 For the vapor flow direction change, the effective angle becomes the average of the corrugation inclination angle and the vertical, i.e., (41° + 90°)/2 ) 65.5°. It should be noted here that, in this case, the bends on both ends are quite short; therefore, there is no significant change in the length of the vapor flow channel. The corresponding definitions, as well as the complex expressions for predicting the loading-point gas load that differ slightly for constant- and variable (total reflux) liquid load situations, can be found elsewhere.16,20,21 To illustrate the relative magnitude of the individual pressuredrop components and the effects due to the change in the corrugation inclination angle, in Table 3 a comparison is given of the predicted values for, respectively, MellapakPlus 752.Y, the same packing with a corrugation inclination angle of 45°, and the conventional counterpart (no bends). As illustrated in Table 3, where the numbers given in the parentheses represent the percentage contribution of the individual components to the total pressure drop, the pressure loss that is related to gas-gas interaction dominates. Namely, reducing the corrugation inclination angle from the common 45° to 41° results in a significant increase in the gas-gas interaction component and, consequently, an almost 40% increase in the total pressure drop. It also adversely influences the pressure drop in the wall zone, whose contribution to the pressure drop due to the change in flow direction is relatively large in small-diameter columns. Therefore, the reduction in the pressure drop that is related to the change in gas-flow direction is not so pronounced. For a

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Figure 1. Photograph of MellapakPlus 752.Y packing elements used in this study and the schematic of a sheet illustrating the corrugations design. Table 2. Geometric Parameters of MellapakPlus 752.Y parameter

value

specific area, ap porosity,  corrugation base length, b corrugation height, h corrugation side, s corrugation sheet height, hpe corrugation angle, R

510 m2/m3 0.975 9.85 mm 6.50 mm 8.16 mm 200 mm 41°

column with a diameter of 1 m, the total pressure drop would be reduced by 10%, and the relative contribution of the change in gas-flow direction would decrease from ∼26% to ∼15%. According to the model, the reduction in the gas-flow angle leads to an improved packing efficiency. This is mainly due to the corresponding increase in the effective gas and liquid velocities, i.e., increased gas-side mass-transfer coefficient and effective specific area, respectively. On the other hand, the hydraulic capacity is governed by the angle of flow direction change. With respect to the conventional configuration, the predicted capacity increase is ∼20%, which is, however, less than that observed in proprietary experiments conducted with the CB/EB system.8,9 As demonstrated in this exercise, the original model anticipates the changes in the inclination of the gas-flow angle within and at the ends of the corrugated sheets. Another practical feature of the Delft model is that it does not include the liquid holdup explicitly, which, being the main

Table 3. Illustration of the Effects of the Corrugation Angle and Form on the Pressure Drop and Efficiency of MellapakPlus 752.Y, as Predicted by the Delft Model for a Column with Internal Diameter of 0.45 m Value parameter σGL σGG σDC σGL + σGG + σDC FGuGe2/2 in Pa dp/dza FGlp HETPa a

41°

45°

45°, no bends

20.3 (19.6%) 57.6 (55.5%) 25.8 (24.9%) 103.7 (100%)

19.0 (22.2%) 43.6 (50.8%) 23.2 (27.0%) 85.8 (100%)

19.0 (21.0%) 43.6 (48.2%) 27.9 (30.8%) 90.5 (100%)

2.14

1.84

1.84

1.39 mbar/m 1.65 m/s(kg/m3)0.5 0.215 m

0.99 mbar/m 1.65 m/s(kg/m3)0.5 0.238 m

1.04 mbar/m 1.42 m/s(kg/m3)0.5 0.238 m

At FG ) 1.25 m/s (kg/m3)0.5.

cause of the excessive pressure drop within the loading region, requires an iterative calculation approach, such as, for instance, that incorporated in the well-known Rocha et al. method.2,3 Indeed, the liquid holdup is an essential design and operating parameter and should be known during packed-column design/ rating considerations. Analogous with pressure drop behavior, the distinction is made between preloading and loading regions. This means that the dynamic holdup of the loading region is considered to be composed of two contributions: the film flow (preloading

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region) and the excess contribution, which is due to additional liquid buildup imposed by increased gas-flow shear force beyond the load point. The buildup of the liquid starts at the points where the liquid drains from the packing elements, i.e., at the transitions to the element below. To account properly for this excess liquid, the expression proposed by Hoffman et al.22 for catalytic packing is adopted here. This is the expression on the right-hand side of eq 4 (shown in Table 1). The pressure drop is that which corresponds to the given conditions calculated using eq 1 of the Delft model. Assuming a uniform liquid distribution, the liquid holdup in the preloading region is calculated using eq 5 (Table 1), i.e., simply as the product of the specific packing area and the constant film thickness. The latter is calculated using the well-known Nusselt formula for laminar falling films (which is a valid assumption for common distillation applications) adapted for inclined surfaces. The effective flow angle of the liquid, RL, is dependent on the dimensions and the inclination angle of the corrugations and can be estimated using a theoretically founded expression,17,20 based on the assumption that only gravity and the macroscopic geometry of corrugations (not the surface texture) affect the flow. The mass-transfer efficiency is expressed in terms of the HETP, i.e., the height of a packed bed equivalent in separation performance to one theoretical plate or equilibrium stage. The HETP is related to the height of an overall gas-side-based transfer unit, HoG, by eq 6 (Table 1), to the extent determined by the value of the characteristic stripping factor, λ (i.e., the ratio of the slopes of the equilibrium and operating lines). According to the resistance-in-series model, the height of the overall transfer unit is composed of individual gas- and liquidside contributions (eq 7 in Table 1), and the contribution of the liquid side is dependent on the stripping factor. Under total reflux conditions (L/G ) 1), the stripping factor is identical to the slope of the equilibrium line (m), which can be determined for a given relative volatility R and composition x, using eq 8. Individual gas- and liquid-side-related transfer units are defined as a quotient of the corresponding superficial velocity and the product of the corresponding mass-transfer coefficient and the effective specific area of the packing. Because masstransfer performance does not seem to be affected by adaptations made in the geometry of corrugations to enable an effective increase in packing capacity, the model predictions are based on correlations used in the Delft model for conventional structured packings.17,20 Sulpak. Sulpak is a proprietary software package that is distributed via the Internet7 to users who are interested in the performance of Sulzer Chemtech packings. The basic model was first published in 19926 and later extended and updated accordingly to include new packing types and sizes. The characteristic packing-specific parameters were obtained over the years from the evaluation of Sulzer proprietary air-water and total reflux experiments that were conducted mainly with the CB/EB system at sub-atmospheric and atmospheric pressures. Additional information on the performance of various Mellapak packings is mainly provided by Fractionation Research, Inc. (FRI), covering different systems in conjunction with a wide operating pressure range.23 For the purpose of this study, Sulpak has been used as available (Version 3.3), applied to the conditions of the airwater and total reflux experiments performed recently in Delft.

Table 4. Physical Properties of the Air-Water System property liquid density liquid viscosity surface tension vapor density vapor viscosity

air/water (average) 999 kg/m3 1.15 × 10-3 Pa s 0.073 N/m 1.2 kg/m3 1.82 × 10-5 Pa s

Experimental Studies The diameter of the Mellapak 752.Y elements was chosen to fit into TU Delft pilot scale columns for air-water hydraulics and total reflux studies, both with an internal diameter of 0.45 m. The flowsheet and a detailed description of the column hydraulics simulation setup can be found elsewhere.24 In short, it is a transparent column composed of Perspex, used in conjunction with air-water as the test system under ambient conditions to measure the pressure drop and the operating liquid holdup as a function of gas load for several constant liquid loads. The average properties of the air-water system are given in Table 4. In the present study, the installed bed height was 1.6 m; i.e., the beds consisted of 8 packing elements rotated to each other, as usual, by 90°, starting from the bottom-packing element oriented perpendicular to the gas inlet device. The liquid distributor was a narrow trough type that contained 10 drip tubes at the periphery and 6 drip tubes in the center, which is an equivalent to 100 drip points per square meter. It was always positioned such that the drip tubes in one line were rotated by 45°, with respect to the sheet orientation of the top element. The distance from the drip tubes to the packing was ∼5 cm. Note that the irrigation density used in this study is well below that recommended by Sulzer for MellapakPlus 752.Y packing. Before a series of measurements, the column was operated at a high liquid load for some period of time to ensure thorough wetting of the packing. The liquid load then was set to the desired flow rate and, after stabilization, the gas load was increased in regular time intervals, from a low value up to the flooding velocity. The temperature of the air leaving the column was registered in regular time intervals and averaged with the temperature of the surroundings to calculate the mean air density during the experiment. This average density was used in conjunction with the superficial column velocity to determine the corresponding gas load,

FG ) uGs(FG)0.5

(in units of m/s(kg/m3)0.5 ) Pa0.5)

i.e., F-factors values over the entire range of operation. The pressure drop over the packed bed was measured using a water-filled U-tube manometer. The operating holdup without gas load was determined indirectly, as a difference in the quantities of the liquid measured with and without a packed bed. The dynamic (operating) holdup of the packed bed was determined from the recorded changes in the level height in the tank. Mass-transfer efficiency studies were conducted using a new total reflux installation assembled recently from main components donated by J. Montz GmbH.25 A general view of the TU Delft equipment is shown in Figure 2. The reboiler is a falling film partial evaporator (20 m2 area), with 1.5-5 bar of steam as the heating medium (1 ton/h steam). Closed-circuit water was used as a cooling medium for vapor condensation in a U-tube-type condenser with an installed heat-transfer area of ∼40 m2. The heart of the installation is a 0.45-m internal diameter column that allows accommodation of beds up to

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Figure 2. Flowsheet of the TU Delft total reflux distillation installation.

2.5 m in height. The bottom portion (sump) of the column was made larger to provide space for a vapor inlet, as well as space for the initial liquid charge (∼0.5 m3). Because of compatibility reasons, a binary cyclohexane-nheptane mixture (CH/n-H) was chosen as the principal test system. Namely, this well-established mixture is one of the recommended systems for total reflux distillation tests,26 and it also is used as the standard test system at FRI and the Separations Research Program (SRP) of the University of Texas at Austin. A parallel run was also made with the methanolwater system (M/W), which was considered to be the most suitable system for evaluating the mass-transfer performance of a new generation of Sulzer catalytic packings (Katapak-SP 12) that contain an Amberlist catalyst, which, in turn, requires an aqueous environment to survive. Note that the sheets of MellapakPlus 752.Y are constitutive components of KatapakSP and that the present study was arranged to provide basecase (reference) data. All test runs were conducted at atmospheric pressure. Initial compositions of the binary mixtures, as well as the packed heights, were chosen to avoid operation in pinch zones of the vapor-liquid equilibrium. For the M/W system, which proved to be more difficult in this respect, the bed height was reduced to 1 m, whereas in the case of the CH/nH system, the bed height was equal to that used in hydrodynamic air-water experiments, i.e., 1.6 m. In both cases, the same type of liquid distributor as that used in the air-water experiments was used. As mentioned previously, the experiments were conducted under total reflux conditions and the entire operation was controlled by an online system interface that was operated on a personal computer. All temperatures, pressures, and flows, which were continuously recorded, were also displayed at corresponding instruments located in a hardware box in the control room. The experimental runs were executed on a daily basis, starting with the flooding condition to ensure thorough wetting of the packing. After a warming-up period of ∼3 h, which was required to get steady-state pressure, temperature, and flow profiles (1 h unchanged), the run was continued at chosen constant vapor loads (F-factor) until the analyses showed no changes in the process parameters (constant pressure,

temperature, and reflux flow) for at least 1 h. For each point, three liquid samples were taken, in intervals of 45 min. The switch to the new point again usually required 1 h to arrive at the steady-state operating conditions. In this way, two sets of data points were collected per experimentation day. The liquid sampling locations, as well as those of the pressure sensors of the pressure drop cells, are also indicated in the flowsheet of the test installation shown in Figure 2. The liquid samples were taken at regular intervals from the reflux return line and from a liquid collector installed directly below the bed, perpendicular to the orientation of the corrugation sheets. The liquid samples of CH/nH were analyzed using gas chromatography (GC). M/W samples were analyzed using Karl Fischer coulometric titration method. Column sump samples were taken at the beginning of each daily run to establish the begin composition. The main feature of having the liquid sampling devices placed in the reflux line and the liquid collecting/return line below the bed is that, by the virtue of their design, they allow a fresh sample to always be obtained. Samples were also taken frequently from the pump supply line, to validate the regular samples collected below the packing. Pressure drop measurement was performed using two calibrated pressure-difference sensors connected in parallel: one for the low- (0-10 mbar), and the other for full-pressure range of (0-100 mbar). The diameter of the collecting lines was large enough to prevent any adverse effect of possible vapor condensation. The measured pressure drops are usually expressed as a function of the vapor load, i.e., the F-factor. Note that, under the total reflux conditions, the superficial liquid and vapor velocities are related through the ratio of the vapor and liquid densities. i.e. uLs ) uGs(FG/FL). Hence, the operating F-factor was determined via the liquid flow rate of the reflux, which was measured using a Coriolis mass-flow meter. The mass-transfer efficiency (i.e., HETP) is not measured directly; it follows from the equation

HETP )

hpb N

(9)

where hpb is the height of the packed bed and N is the number

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Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007 Table 5. Physical Properties of the Cyclohexane-n-Heptane (CH/nH), Methanol-Water (M/W), and Chlorobenzene-Ethylbenzene (CB/EB) Systems (Average, Rounded Values are Given) Value

Figure 3. Equilibrium curves of the test systems considered in this study at atmospheric pressure.

property

CH/nH

M/W

CB/EB

molecular weight pressure temperature liquid density liquid viscosity liquid diffusivity surface tension vapor density vapor viscosity vapor diffusivity relative volatility stripping factor liquid loada

92 kg/kmol 1.013 bar 87 °C 658 kg/m3 2.87 × 10-4 Pa s 4.35 × 10-9 m2/s 0.0154 N/m 3.00 kg/m3 8.09 × 10-6 Pa s 4.53 × 10-6 m2/s 1.70 0.934 18.94 m3/(m2 h)

25 kg/kmol 1.013 bar 73 °C 854 kg/m3 3.17 × 10-4 Pa s 7.25 × 10-9 m2/s 0.0389 N/m 1.15 kg/m3 1.21 × 10-5 Pa s 3.70 × 10-5 m2/s 3.65 0.675 9.04 m3/(m2 h)

110 kg/kmol 0.96 bar 132 °C 860 kg/m3 2.66 × 10-4 Pa s 6.90 × 10-9 m2/s 0.0186 N/m 3.22 kg/m3 9.41 × 10-6 Pa s 5.34 × 10-6 m2/s 1.12 1 15.02 m3/(m2 h)

a

of equivalent theoretical plates (equilibrium stages), which, in turn, is related to the measured top and bottom compositions through relative volatility. For the temperature and composition ranges, where the relative volatility can be considered to be practically constant, the corresponding number of theoretical stages is calculated using the well-known Fenske equation for total reflux conditions:

N)

ln{[xD/(1 - xD)][(1 - xB)/xB]} ln R

(10)

where x is the mole fraction of the light component, whereas the subscripts B and D denote bottoms and distillate, respectively. For the relative volatility (R), the geometric average of the top and bottom values is usually considered to be representative. This equation is valid for an almost-ideal CH/nH system. Unfortunately, the nonideal M/W system is much more complicated in this respect. Because of a strong dependence of relative volatility on the temperature and the composition, the values of the relative volatility increase in this case, from, for example, 2 in the upper part to >5 in the bottom portion of the bed. To avoid possible inaccuracies that are due to differences in the profiles of relative volatility, the number of theoretical stages was calculated in both cases using an analytical equivalent of the graphical McCabe-Thiele method. Experimental equilibrium curves for atmospheric pressure were taken for this purpose from the Onken and Arlt booklet that describes the recommended test mixtures for distillation.26 The corresponding y-x plots for the CH/nH and M/W systems are shown in Figure 3, including, for reference, the ideal CB/EB system, which is the base test system at Sulzer. In fact, the number of equilibrium stages was obtained by counting the stages accommodated between the measured top and bottom compositions. The obtained HETP values can be transformed to corresponding values of the overall height of the mass transfer unit (HoG), using the inverse form of eq 6. When the equilibrium and operating lines are straight and parallel, HETP ) HoG; however, for straight equilibrium lines with a slope different from that of the operating line, these two packing efficiency measures can differ considerably, depending on the system. Usually, both the HETP or HoG and the pressure drop observed during total reflux experiments are expressed graphically as a function of the vapor load (F-factor). This is associated with another important issue, i.e., the choice of the representative F-factor. Generally, because of the differences in temperature

At FG ) 2 m/s(kg/m3)0.5.

and composition and, consequently, vapor densities, the F-factors usually differ for the bottom and top conditions. This is more pronounced in the case of the M/W system, because of a much stronger mixture composition effect. For the almost-ideal CH/ nH system, FRI prefers the middle of the column, and SRP prefers the bottom conditions, the latter being the most beneficial, with respect to both efficiency and pressure drop and, consequently, capacity. Namely, this is mainly due to the larger effect of composition than that of temperature on the vapor density. The corresponding increase in the vapor density is so large that it dominates over the certain decrease in the velocity of vapor. For the CH/nH system, the effect is rather limited (FG,bottom/FG,top ) 1.1). However, the effect of operating conditions on the F-factor of the M/W system is much more pronounced (FG,bottom/FG,top ) 1.28). To provide a consistent basis for comparison of two different systems, in this study, averaged values of the F-factor are used, based on the vapor and liquid densities of a 50/50 mixture at the average temperature of the bed. Table 5 contains a summary of the average values of all relevant physical properties used for the interpretation of experimental data and/or model validation purposes. Note that, besides the already mentioned relative volatility, the aqueous and organic systems also differ considerably in regard to vapor diffusivity, vapor density, and surface tension. It should be noted that rather a low vapor density means, for an aqueous system, a significantly lower liquid load at the same F-factor. Also interesting is the difference in the stripping factor, i.e., the slope of the equilibrium line, indicating that, with the decreasing relative volatility, the difference between the HETP and HoG values decreases, to zero in the case of the CB/EB system. Results and Discussion Air-Water Hydrodynamics. Figure 4 shows a comparison of measured and calculated liquid holdup values for several liquid loads. The prediction of the Delft model agrees very well with the measurements performed without the presence of the gas flow. The accuracy of the simple, theoretically founded eq 5 is strikingly good. At very large liquid loads, well beyond those encountered in applications of MellapakPlus 752.Y, Sulpak overestimates by ∼10%. Figure 5 shows the measured holdup and the holdup predicted by eq 4, expressed as a function of gas loads with the liquid load as a parameter. As expected, the measured holdup increases as the liquid load increases, and the effect of the increasing gas load becomes more pronounced at higher liquid loads. The change in the slope of the curve at higher gas loads indicates the point of the onset of loading, i.e.,

Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007 889

Figure 4. Dynamic liquid holdup as a function of the liquid load (uGs ) 0 m/s), measured vs predicted.

Figure 5. Dynamic liquid holdup as a function of the gas load and the liquid load, measured vs predicted.

excessive buildup of the liquid, which usually starts at the bottom portions of the packing elements, i.e., at the transitions between packing elements. This phenomenon has been observed in an experimental study that was performed with Mellapak 250.Y and Mellapak 250.X, using a γ-ray technique at Sulzer in the beginning of the 1990s,27 as well as via computed X-ray tomography, in conjunction with a bed of plastic Mellapak 250.Y.28 As expected, as the liquid load increases, the point of onset of loading shifts to lower gas loads, indicating the extent of reduction in the capacity of packing with increasing liquid load. Because the onset of loading is accompanied by a sharp increase in the pressure drop, which is visible in the plot shown in Figure 6, actually these two points should coincide. However, through careful observation of an operating experiment, it can be observed that the pressure drop starts to increase somewhat earlier than a (visible) buildup of liquid is initiated. Also, the hold-up curves shown in Figure 5 indicate that the new eq 5 that is applied in the Delft model overpredicts the loading point, to some extent. Regarding the observed effect of the gas flow close to the loading point, the predictions in this region are somewhat optimistic, in the case of high liquid loads. In regard to the rather low liquid loads that are involved in industrial applications of MellapakPlus 752.Y, this is practically not a deficiency. Figure 6 also shows a comparison of measured and predicted (by Delft model) dry and wet pressure drops, as a function of the F-factor, for several constant liquid loads. As expected, the pressure drop increases as both the gas load and liquid load

Figure 6. Pressure drop as a function gas and liquid loads, measured vs predicted (Delft model).

increase. The change in the slope of the pressure drop curves indicates the position of the loading point. The latter one shifts to lower gas load with increasing liquid load, analogous to the liquid hold-up behavior. The model slightly overpredicts the dry pressure drop and the wet pressure drops are perfectly reproduced in the preloading region. At the highest liquid load, the prediction of the loading point is too optimistic. However, this must not be considered as a deficiency, because, as mentioned previously, this packing is not used in applications with such a high liquid load. Certainly, a bend to vertical significantly reduces the pressure drop associated with the transitions between packing elements. However, because of a sharper corrugation angle, the effective velocity of gas or vapor in the flow channels of MellapakPlus 752.Y is larger than usual, which implies a certain increase in the pressure drop, which is due to friction at the phase interface. Because of a sharper flow angle, an even greater increase in the pressure drop comes from a more-intensive interaction of gas flows at the plane formed at each crossing of the flow channels. In fact, much of the gain in the pressure drop that is due to a smooth transition between packing elements is lost, because of the deteriorating effects of the reduction in the corrugation inclination angle. In other words, these two effects compensate each other to some extent, and the main gain is related to the fact that the new smooth transition geometry allows a smooth drainage of liquid, i.e., a significantly delayed loading and, consequently, a higher capacity than it is the case with the conventional version of this packing. Figure 7 shows that the Sulpak predictions are similar in trend and achieved accuracy in the preloading region. With increasing liquid load, Sulpak has a tendency to become conservative. Total Reflux Distillation. Figure 8 shows the measured pressure drop and efficiency (expressed as HETP and HoG, respectively) of the MellapakPlus 752.Y, as a function of the F-factor for the CH/nH and M/W systems, respectively. Interestingly, both the capacity and the efficiency of the packing are better in the case of the aqueous system. The lower pressure drop and, consequently,the larger capacity in the case of the M/W system is mainly due to significantly lower liquid loads at the same F-factor (a factor of >2, according to Table 5). This is an important difference that affects the results more pronouncedly than, for instance, the difference in the surface tensions of the two systems. Certainly, the larger surface tension of the aqueous system is a reason more effort is needed for vapor to entrain the liquid. This occurs gradually, and efficiency follows the trend of the pressure drop. The loading region is wider, and efficiency

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Figure 7. Pressure drop as a function gas and liquid loads, measured vs predicted (Sulpak).

Figure 8. Measured pressure drop and packing efficiency as a function of the column load (F-factor) for cyclohexane/n-heptane and methanol/water systems.

gradually deteriorates and is finally lost at a pressure drop just above 10 mbar/m. Also, in the case of the CH/nH system, the efficiency and hydraulics coincide in behavior. However, upon reaching the critical loading, which occurs at a pressure drop of 6 mbar/m, both the pressure drop and efficiency are departing suddenly, which indicates the onset of flooding. Interestingly, the organic system exhibits a rather stable masstransfer performance until the point of the onset of flooding, whereas the efficiency of the aqueous system has a tendency to decrease slowly with increasing F-factor. In the loading region, the deterioration in the efficiency of the aqueous system is more pronounced, which leads to an efficiency similar to that of the CH/nH system. Dashed thin lines shown in Figure 8, which follow the trend of the HETP curves, are the corresponding HoG curves. Obviously, the difference in the absolute values is quite small in the case of the CH/nH system, which is due to a stripping factor value that approaches unity. Much lower stripping factor values cause a larger difference in the case of the M/W system. Regarding the surface tension gradient associated with the M/W system, which is notorious for everlasting suspicions, with respect to potential detrimental effects on the efficiency,29,30 it is surprising that, in the preloading region, the performance of the packing is better, in conjunction with the aqueous system. Nevertheless, scarce experimental evidence obtained using sufficiently large equipment (d ) 0.3 m) indicates that a structured packing (220 m2/m3) can perform better with the M/W

Figure 9. Comparison of measured and predicted pressure drop at total reflux conditions for methanol/water system.

system than with a methanol-isopropanol mixture.5 Evaluation of the total reflux and continuous distillation experiments performed recently with a mixture of methanol, ethanol, and water in a 0.22-m-internal-diameter column, equipped with a 250 m2/ m3 packing, indicated that even a significant fraction of water does not introduce any performance-deteriorating effect.31 A plausible explanation for such a good efficiency as observed with the M/W system in the present study may be found in a beneficial effect of the rather short length of the bed used in this case (1 m). Namely, as demonstrated by Stichlmair and Fair,32 a good initial distribution, which is observed after one element translated into that inherent to the size of the corrugations and the nature of surface, remains preserved within the first 5-6 elements. In other words, from this point onward, it starts to deteriorate significantly, adversely affecting the efficiency. This means that a 1.6-m bed, such as that used in the case of the CH/nH system, would produce a somewhat larger HETP value. However, with a bed of 1.6 m, almost pure methanol was obtained at the top, which made data analysis and interpretation unreliable. On the other hand, as mentioned previously, the efficiency in the case of the CH/nH system is practically constant, i.e., unaffected by the F-factor until the point of onset of flooding. The average HETP value is somewhat higher than anticipated for the packing with such a large surface area; however, the CH/nH system is notorious for this phenomenon. Namely, some other tests with low relative volatility systems, such as the CB/ EB system and cis/trans-decaline produce generally lower HETP values than the CH/nH system under the same conditions. Model Validation. According to Figure 8, the pressure drop of the M/W system is significantly lower, which is attributed mainly to the liquid load being a factor of 2 less than that at the same F-factor. Predictions of the Delft model and Sulpak are compared with the measured curve in Figure 9. Strikingly, in the preloading region, Sulpak and the Delft models produce almost equal values. Both methods are conservative in preloading and loading regions and show a steeper increase in pressure drop than that observed in the loading region. The Sulpak result is more similar to a smooth transition around the loading point, which is more pronounced and occurs at somewhat lower vapor load than in the case of the Delft model. Figure 10 indicates that, for the CH/nH system, the pressuredrop prediction of the Delft model coincides again with that of Sulpak in the preloading region, closely approaching the measured curve. Both methods follow the trend of the pressure drop in the loading region perfectly, but underpredict the loading point. Sulpak is slightly more conservative in this respect.

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Figure 10. Comparison of measured and predicted pressure drop at total reflux conditions for cyclohexane/n-heptane system.

Figure 11. Comparison of measured and predicted pressure drop at total reflux conditions for chlorobenzene/ethylbenzene (CB/EB) system.

Figure 12. Comparison of measured and predicted efficiencies at total reflux conditions for cyclohexane/n-heptane system.

Figure 11 shows a slightly optimistic but still quite good performance of the Delft model, with respect to the pressure drop of the CB/EB system, as measured in the 1-m internal diameter column at Sulzer. As expected, in this case, the Sulpak result is more similar to the measurement, with the exception of the loading region, where the increase in the pressure-drop curve is stronger than that observed. Figure 12 indicates that the efficiency prediction of the Delft model for the CH/nH system is remarkably good. Note that the Delft model prediction indicated by prediction curve a in Figure

Figure 13. Comparison of measured and predicted efficiencies at total reflux conditions for methanol/water system.

Figure 14. Comparison of measured and predicted efficiencies at total reflux conditions for the CB/EB system.

12 includes the correction term for the certain loss of efficiency, as generally experienced with larger-specific-area packings.20 Because this is not the case with MellapakPlus packings,8 the correction term is actually not needed. In other words, the correction term should be omitted. In that case, the well-known Onda correlation adapted for structured packings,17,20 reduced by some 10%, corresponding to the area loss due to perforations is used as the expression for the prediction of the effective specific area. This delivers the curve with open squares, which is denoted as prediction curve b in the figure; i.e., this is a somewhat optimistic prediction. Also, note here that, in the Delft model, there are no provisions to account for the complex behavior as experienced within the loading region. This simplification is justified by a general experience that structured packings in the case of organic mixtures have a tendency to preserve or improve their efficiency in the loading region, so that the predictions based on the preloading region are on the safe side. Figure 13 compares the efficiency predictions of the Delft model with the measured values for the M/W system. The trend of the Delft model curve is good, and the extent of the overprediction in the preloading region is just within common safety margins, but slightly optimistic in the upper range of the loading region. Note that, in this case, it is assumed that the aqueous system will bridge over the holes in the surface; i.e., it would use the area occupied by holes in the extent predicted by the modified Onda correlation.17,20 Figure 14 shows that the Delft model is on the safe side but close enough to the observed curve for the CB/EB system, which

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is the reference system used by Sulzer and, therefore, reproduced perfectly by Sulpak. Note that the HETP values available in the Sulpak are not predicted but practically identical to the measured curve for the CB/EB system. For other systems, Sulzer recommends values based on an in-house estimation method. An amazingly good performance of the generalized Delft model, in this case, is the best recommendation for this general method requiring no packing-specific, empirically fitted parameters to be used widely as a predictive tool for conceptual design work that is suitable for both new designs and retrofits of existing packed columns. Conclusions The hydraulic and mass-transfer performance of a commercial high-capacity structured packing with a large specific surface area (MellapakPlus 752.Y) was evaluated experimentally using pilot-scale installations available at TU Delft. Air/water experiments produced results similar to that observed in similar tests performed at Sulzer. With the cyclohexane/n-heptane system, MellapakPlus 752.Y exhibited good and stable performance until reaching the point of the onset of flooding. A less-stable but very good performance was obtained with the methanol/water system, which may be attributed to the fact that, within the short bed used, there was no chance for liquid maldistribution to develop to a performance-deteriorating level. With the appropriate model adaptations made to account properly for the geometry characteristics of MellapakPlus packings, the Delft model seemed to be capable of producing reliable and astonishingly good predictions; these results also included data from the chlorobenzene/ethylbenzene system that was produced in a large diameter column used at the Sulzer laboratory. With this in mind, the Delft model may be considered as a reliable tool to predict the performance of conventional and highcapacity structured packings. Because it does not require any empirical, packing-specific constant, the Delft model provides a good guidance for the purposes of the conceptual design and retrofitting of packed columns that contain corrugated-sheet structured packings. Acknowledgment The authors are thankful to Sulzer Chemtech for providing the packing and permitting us to publish the data. Sander Balkenende and F. van Dijk performed the measurements and data evaluation and interpretation with, respectively, the organic and aqueous systems, as a part of their graduate thesis work. Nomenclature ap ) geometric specific area of packing (m2/m3) ae ) effective (interfacial) specific area (m2/m3) dhG ) hydraulic diameter for gas flow (m) FG ) vapor load (boil up), i.e., F-factor (m/s (kg/m3)0.5) Fload ) pressure drop enhancement factor G ) molar flow rate of gas (vapor) (kmol/s) g ) gravitational acceleration (m/s2) HETP ) height equivalent to a theoretical plate (m) HG ) height of the gas-side transfer unit (m) HL ) height of the liquid-side transfer unit (m) HoG ) height of an overall gas-side transfer unit (m) hL ) liquid holdup (m3/m3) hpb ) height of the packed bed (m) kG ) gas-side mass-transfer coefficient (m/s) kL ) liquid-side mass-transfer coefficient (m/s)

L ) molar flow rate of liquid (kmol/s) L/G ) slope of the operating line m ) slope of the equilibrium line N ) number of equilibrium stages or theoretical plates uGs ) superficial gas velocity (m/s) uGe ) effective gas velocity (m/s) uLs ) superficial liquid velocity (m/s) x ) mole fraction of the light component in the liquid phase y ) mole fraction of the light component in the vapor phase Greek Letters R ) corrugation inclination angle (°) R ) relative volatility RL ) effective liquid flow angle (°) ∆p/∆z ) pressure drop (mbar/m) δL ) liquid film thickness (m)  ) void fraction (porosity) ζ ) flow resistance coefficient λ ) stripping factor; λ ) m/(L/G) µL ) liquid viscosity (Pa s) FG ) gas (vapor) density (kg/m3) FL ) liquid density (kg/m3) Subscripts B ) bottoms D ) distillate G ) gas or vapor phase DC ) direction change GG ) gas-gas interaction GL ) gas-liquid interaction L ) liquid phase load ) refers to the loading region lp ) loading point preload ) refers to the preloading region Literature Cited (1) Stichlmair, J.; Bravo, J.; Fair, J. R. General Model for Prediction of Pressure Drop and Capacity of Countercurrent Gas/Liquid Packed Columns. Gas Sep. Purif. 1989, 3, 19. (2) Rocha, J. A.; Bravo, J.; Fair, J. R. Distillation Columns Containing Structured PackingssA Comprehensive Model for their Performance. 1. Pressure Drop. Ind. Eng. Chem. Res. 1993, 32, 641. (3) Rocha, J. A.; Bravo, J.; Fair, J. R. Distillation Columns Containing Structured PackingssA Comprehensive Model for their Performance. 2. Mass Transfer Model. Ind. Eng. Chem. Res. 1996, 35, 1660. (4) Billet, R.; Schultes, M. Prediction of Mass Transfer Columns with Dumped and Arranged Packings. Updated Summary of the Calculation Method of Billet and Schultes. Trans. Inst. Chem. Eng., Part A: Chem. Eng. Res. Des. 1999, 77, 498. (5) Xu, Z. P.; Afacan, A.; Chuang, K. T. Predicting Mass Transfer in Packed Columns Containing Structured Packings. Trans. Inst. Chem. Eng., Part A: Chem. Eng. Res. Des. 2000, 78, 91. (6) Spiegel, L.; Meier, W. A Generalized Pressure Drop Model for Structured Packings. Inst. Chem. Eng. Symp. Ser. 1992, 128, B85. (7) Sulpak, www.sulzerchemtech.com. (8) Kessler, A.; Moser, F.; Meier, W. MellapakPlus: A New Generation of Structured Packings. Presented at the AIChE 1999 Annual Meeting, October 31-November 5, 1999, Dallas, TX, Paper No. 32g. (9) Pilling, M.; Spiegel, L. Design Characteristics and Test Validation of High Performance Structured Packing, Preprints of the Distillation Symposium. Presented at the AIChE 2001 Annual Meeting, Reno, NV, November 2001, Paper No. 64. (10) Spiegel, L.; Meier, W. Distillation Columns with Structured Packings in the Next Decade. Trans. Inst. Chem. Eng., Part A: Chem. Eng. Res. Des. 2003, 81, 39. (11) Brunazzi, E.; Paglianti, A.; Spiegel, L.; Tolaini, F. Hydrodynamics of a gas-liquid column equipped with MellapakPlus packing. In Proceedings of the International Conference on Distillation & Absorption, Baden-Baden, Germany, September 30-October 2, 2002, Paper No. 6.17. (CD-ROM, ISBN 3-93138.4-37.-3.)

Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007 893 (12) Billingham, J. F.; Lockett, M. J. Development of a New Generation of Structured Packings for Distillation. Trans. Inst. Chem. Eng., Part A: Chem. Eng. Res. Des. 1999, 77, 583. (13) Olujic´, Zˇ .; Jansen, H.; Kaibel, B.; Rietfort, T.; Zich, E. Stretching the Capacity of Structured Packings. Ind. Eng. Chem. Res. 2001, 40, 6172. (14) Bender, P.; Moll, A. Modifications to Structured Packings to Increase their Capacity. Trans. Inst. Chem. Eng., Part A: Chem. Eng. Res. Des. 2003, 81, 58. (15) Olujic´, Zˇ . Development of a Complete Simulation Model for Predicting the Hydraulic and Separation Performance of Distillation Columns Equipped with Structured Packings. Chem. Biochem. Eng. Q. 1997, 11, 31. (16) Olujic´, Zˇ .; Kamerbeek, A. B.; de Graauw, J. A. Corrugation Geometry Based Model for Efficiency of Structured Distillation Packing Chem. Eng. Process. 1999, 38, 683. (17) Fair, J. R.; Seibert, A. F.; Behrens, M.; Saraber, P. P.; Olujic´, Zˇ . Structured Packing PerformancesExperimental Evaluation of Two Predictive Models. Ind. Eng. Chem. Res. 2000, 39, 1788. (18) Olujic´, Zˇ .; Seibert, A. F.; Fair, J. R. Influence of Corrugation Geometry on the Performance of Structured Packings: an Experimental Study. Chem. Eng. Process. 2000, 39, 355. (19) Olujic´, Zˇ .; Seibert, A. F.; Kaibel, B.; Jansen, H.; Rietfort, T.; Zich, E. Performance Characteristics of a New High Capacity Structured Packing. Chem. Eng. Process. 2003, 42, 55. (20) Olujic´, Zˇ .; Behrens, M.; Colli, L.; Paglianti, A. Predicting the Efficiency of Corrugated Sheet Structured Packings with Large Specific Surface Area. Chem. Biochem. Eng. Q. 2004, 18, 89. (21) Verschoof, H. J.; Olujic´, Zˇ .; Fair, J. R. A General Correlation for Predicting the Loading Point of Corrugated Sheet Structured Packings. Ind. Chem. Eng. Res. 1999, 38, 3663. (22) Hoffman, A.; Noeres, C.; Gorak, A. Scale-up of reactive distillation columns with catalytic packings. Chem. Eng. Sci. 2004, 43, 383. (23) Fitz, C. W.; Kunesh, J. G.; Shariat, A. Performance of Structured Packing in a Commercial-Scale Column at Pressures of 0.02-27.6 bar. Ind. Eng. Chem. Res. 1999, 38, 512.

(24) Behrens, M.; Saraber, P. P.; Jansen, H.; Olujic´, Zˇ . Performance characteristics of a monolith-like structured packing. Chem. Biochem. Eng. Q. 2001, 15, 49. (25) Olujic´, Zˇ .; Kaibel, B.; Jansen, H.; Rietfort, T.; Zich, E.; Frey, G. Distillation Column Internals/Configurations for Process Intensification. Chem. Biochem. Eng. Q. 2003, 17, 301. (26) Onken, U.; Arlt, W. Recommended Test Mixtures for Distillation Columns, Second Edition; Institution of Chemical Engineers: Rugby, Warwickshire, England, 1990. (27) Suess, P.; Spiegel, L. Holdup of Mellapak Structured Packings. Chem. Eng. Process. 1992, 31, 119. (28) Marchot, P.; Toye, D.; Pellser, A.-M.; Crine, M.; L’Homme, G. L.; Olujic´, Zˇ . Liquid Distribution Images on Structured Packing by X-Ray Tomography. AIChE J. 2001, 47, 1471. (29) Yang, N. S.; Chuang, K. T.; Afacan, A.; Resetarits, M. R.; Binkley, M. J. Improving the Efficiency and Capacity of Methanol-Water Distillation Trays. Ind. Chem. Eng. Res. 2003, 42, 6601. (30) Proctor, S. J.; Biddulph, M. W. Marangoni Effects in Packed Columns. In Preprints of the Topical Conference on Separation Science and Technologies (Part I), AIChE Annual Meeting, Los Angeles, CA, November 17-19, 1997, p 183. (31) Mori, H.; Ibuki, R.; Taguchi, K.; Futamura, K.; Olujic´, Zˇ . ThreeComponent Distillation Using Structured Packings: Performance Evaluation and Modelling. Chem. Eng. Sci. 2006, 61, 1760. (32) Stichlmair, J. G.; Fair, J. R. DistillationsPrinciples and Practices; Wiley-VCH: New York, 1998.

ReceiVed for reView October 14, 2005 ReVised manuscript receiVed October 20, 2006 Accepted November 13, 2006 IE051146F